The present invention relates to a tire mold, of which a tire tread molding section is divided into a plurality of segments.
In molding a pneumatic tire, there is used a metallic mold, of which a tread molding section for molding of a tire tread comprises a plurality of segments (called sector in some cases) divided in a tire circumferential direction. The tread molding section is usually divided into 7 to 13 segments and the respective segments are provided movably in a radial direction about a tire rotation axis. At the time of tire molding, a mold is closed by moving the respective segments inward in a tire radial direction in a state, in which a green tire is arranged inside the tread molding section. Then a tire is subjected to vulcanizing and molding in the mold.
With a tread molding section 100 of this kind, ridges 102 are provided on respective segments 104 to mold transverse grooves on a tread surface as shown in FIG. 9. Conventionally, all the ridges 102 are formed to project in a direction toward a center Om of the segment 104, that is, perpendicular to a mold surface 105 of the segment 104. Therefore, while a direction, in which a ridge 102C positioned centrally of the segment 104 projects, agrees with a direction M of movement of the segment 104, an angular divergence between a direction, in which the ridge 102 projects, and the direction M of movement of the segment 104 increases as it goes toward ends of the segment 104 from a center thereof. That is, in FIG. 9, an angle θ1 of divergence for the ridge 102E at the segment end is larger than an angle θ2 of divergence for the ridges 102 disposed toward the center.
When a mold having such tread molding section is used to mold a tire, in particular, in the case where vulcanizing and molding are carried out in a state, in which a green tire is supported on a rigid core without the use of any bladder, the following problem is caused because the green tire is approximately the same in outside diameter as a product tire.
That is, in this case, at the time of mold closing, respective segments of the tread molding section are moved inward in a radial direction whereby mold surfaces of the respective segments are pushed against a surface of the green tire. At this time, in particular, the ridges 102E positioned at the segment ends are large in angular divergence between directions of projection thereof and the direction M of movement of the segment 104. Therefore, as the ridges 102E enter a green tire T, rubber of the green tire T is pushed toward ends of the segment 104 from near the center thereof. Such movement of rubber causes rubber to be interposed between adjacent segments in the course of mold closing whereby bite is caused. Also, movement of rubber makes a tread non-uniform in thickness in a circumferential direction. Thereby, adverse influences are produced on uniformity such as RFV (Radial Force Variation) higher order components conformed to the number of division of segments.
In order to solve such problem, U.S. Pat. No. 7,201,570 B2 proposes a construction, in which a plurality of pieces as circumferentially divided are provided on respective segments, pieces disposed centrally of the segments are fixed, and other pieces are made circumferentially movable. In this case, the problem described above is dissolved but the mold becomes complex in structure to be responsible for an increase in manufacturing cost.
JP-A-2003-039436 discloses that a plurality of segments are divided into two groups, timing, in which the segments are closed, is composed of two stages, and segments are closed while coming into slide contact with segments having been closed previously, thereby achieving a decrease in bite. However, this method does not dissolve shift of rubber caused by ridges, which mold transverse grooves.
Also, JP-A-2002-347033 discloses that the number of segments, which is usually 8 to 9, is made, for example, 16 to 18 so as to meet with a predetermined relation, thus achieving a decrease in bite of a green tire. In this case, however, for a green tire being substantially the same in outside diameter as a product tire, it is necessary to increase the number of segments to substantially the same number as the number of pitches of a tread pattern in order to reduce shift of rubber caused by those ridges, which mold transverse grooves, to solve the problem described above, and so a vast equipment investment is required.